The present invention relates to distance determination of RF tags, used in RF identification systems, and more particularly, to a method and system for distance determination of RF tags and its applications, based on measuring the round trip delay.
RF identification (RFID) systems are used to track objects, animals and/or people in a large range of applications. RFID systems are radio communication systems that communicate between a radio transceiver, called a reader, and a number of inexpensive devices called Tags. An RFID system generally includes a plurality of tags which are attached to objects being monitored and one or more readers which are used to communicate with the tags. An encoder is optionally used to program the tags with unique identification information.
One of the tasks commonly performed in RFID systems is determining which tags are located in the vicinity of a reader. For example, the reader may be located near a gate and it is desired to know which tags pass through the gate. Generally, the reader is transmitting broadcast signals in its vicinity and the tags receiving the broadcast signal are responding to the broadcast signal. Moreover, it is required to prevent occurrence of collisions between the response signals of different tags. Various methods have been suggested for preventing collisions. Exemplary collision overcoming methods are described in U.S. Pat. Nos. 5,777,561, 6,265,962 and 6,377,203.
In U.S. Pat. No. 5,777,561, issued to Chieu et al, there is disclosed a method for selecting groups of radio frequency RF tags for communication between a base station and the tags. The tags are selected into groups according to a physical attribute of the signal transmitted by the tags to the base station, or according to the physical response of the tags to a physical attribute of the signal transmitted from the base station to the tags.
In U.S. Pat. No. 6,265,962, issued to Black el al, there is disclosed a method for reading one or more RFID transponders that are active and capable of communicating, comprising: providing a carrier signal; detecting the presence of at least one transponder, the detecting including using a demodulator and modifying the carrier signal by suppressing the carrier signal for a predetermined number of clock cycles; receiving data from all active transponders, the receiving including receiving the data in groups of one or more bits and checking the validity of each group of data as the group is received; determining whether a valid data transmission has been received by detecting inability to compute a proper synchronization word, a proper CRC, or a proper word length; in response to determining an invalid data transmission, modifying the carrier signal to inform all active transponders that there was an incomplete read; and transmitting the complete data for each transponder to a computer system for processing.
In U.S. Pat. No. 6,377,203, issued to Doany, there is disclosed a method for simultaneously reading a serial number from multiple colliding RF signals from RF identification tags by employing a primary communication channel and multiple secondary channels. A locator or reader unit interrogates the tags, which respond with an RF signal. Upon receiving multiple signals, the locator or reader unit commands the RFID tags and requires them to transmit another response, which is transmitted in one of the secondary channels based on a portion of their unique serial identification number, thereby forcing the tags to sort in the secondary channels. These secondary channels are assigned using a portion of the unique serial identification numbers for the tags. The reader then detects an acknowledgment in the occupied secondary channels and commands a tag in a particular channel to move to the primary channel. Once the signal is transmitted in the primary channel, the reader reads the identification number. If there is further collision, the collided tags are returned to the secondary channel and sorted again using a different portion of their ID number. This process continues until all the tags are sorted and identified. Once the tags are all sorted, the tags are assigned yet another channel distinct from the secondary channels that are used for sorting.
The most significant limitation of each of the just described collision overcoming methods of U.S. Pat. Nos. 5,777,561, 6,265,962 and 6,377,203, is that all of these collision overcoming methods are time consuming. Moreover, the required time greatly increases with the number of tags participating in the collision.
In one collision avoiding method used with the ALOHA protocol (described below), transmitters re-transmitting after a collision, wait a pseudo-random period before re-transmitting the signal, so that the chance of a collision occurring between the retransmitted signals is small. The random periods are longer than the transmitted signals, such that response signals of transmitters selecting different random delays do not collide. ALOHA protocol is implemented according to the following two principles: (a) If a station has data to transmit, transmit it. (b) If transmissions collision from different stations occurs, wait a random time and transmit the data again. Additional information regarding the ALOHA protocol can be found in “Computer Communications”, Principles and Business Applications, by Andy Sloane, Mcgraw Hill ISBN 0 07 709443 3.
In some applications, it is important to know that the identified tag is located within a certain distance from the reader. For example, the identification of the tag may be required in order to open a door to an access-limited area If a tag remote from the reader is identified by the reader, the door may be opened to an unauthorized individual.
One solution to that distance measurement problem is to limit the transmission range of the reader. This, however, may cause some tags not to be identified due to their orientation and/or interference.
In U.S. Pat. No. 6,335,685, issued to Schrott el al, there is disclosed an apparatus and method for locating containers and contents of containers using radio frequency tags. A base station system for communicating with radio frequency tags attached to one or more objects. The base station has computers having CPUs and memories. A separate position detector determines the position of the tags within a time increment and within a field of the base station. A communication process, executed by the CPUs, reads information from the tags within the time increment and associates the position determined with the information of the respective tag in the memories. The method features a movable base station antenna providing a narrow tag interrogation beam is used as the position detector. The antenna of the reader is designed to have rotational motion to allow for scanning in a vertical plane. Scanning accomplished as a function of position with the antenna scanning vertically while the object moves horizontally. In that mode of scanning, each tag is scanned individually as it passes the base station antenna so that the combination of horizontal object motion with vertical scanning results in a xy coordinate associated with each tag readout. The horizontal motion can be determined by knowing the velocity of the object.
However, the just described method suggesting using a separate position detector to detect the position of the tag identified by the reader is notably limited because it adds to the cost and complexity of the RFID system. In addition, the position detection is performed after the tags are identified in a separate stage, which adds to the time required for the system operation.
In U.S. Pat. No. 5,977,913, issued to Christ, there is disclosed a method for locating an object within a predetermined area comprising the steps of: (a) sensing a signal output by the object using a plurality of sensors disposed within the predetermined area; (b) forwarding a plurality of received signal levels from the plurality of sensors to a central processor; (c) comparing the plurality of received signal levels with a plurality of reference values to determine a location of the alarm signal, wherein step (c) comprises performing a best fit analysis of the plurality of received signal levels with the plurality of reference values; (d) transmitting one at a time a plurality of reference transmission signals; (e) measuring a received signal level output from each of the plurality of sensors after each transmission.
However, position detection systems, such as described in U.S. Pat. No. 5,977,913, are complex and generally require a plurality of readers in different locations.
In U.S. Pat. No. 6,362,738, issued to Vega, there is disclosed a RFID reader containing a detector circuit for detecting the presence of a signal carrier frequency transmitted by the transponder in response to a signal from the reader. The detector circuit has a resonator circuit which is connected to a receiver electrode. The resonator comprises a piezoelectric element with a high quality factor ‘Q’ at the resonant frequency to enhance sensitivity. The alarm carrier signal is rectified and fed to either a peak detector or an envelope detector circuit. A voltage source generates a voltage threshold to allow for operating range adjustment. A comparator compares both voltages and generates an alarm signal if the voltage signal reaches the threshold voltage. The RFID further having the capability to read the RFID contents.
The system disclosed in U.S. Pat. No. 6,362,738 limits the range of the tags it identifies by relating only to signals whose voltage level is above a predetermined threshold. The threshold may be user-adjusted in order to allow for different ranges of operation. The use of power thresholds is inaccurate, as the power may depend on the orientation of the tag and/or on obstructions between the tag and the reader. In addition, the system of U.S. Pat. No. 6,362,738 does not disclose a solution for resolving collisions when a plurality of tags respond together.
To date, the inventor is unaware of prior art teaching of a method of communicating between a reader and one or more terminal stations features receiving from a plurality of terminal stations a plurality of RF response signals, determining for the plurality of the received response signals a round trip delay, and thereby determining, for each of the response signals, a distance between the reader and the terminal station from which the response signal was received.
Moreover, the inventor is unaware of prior art teaching of a method of communicating between a reader and one or more terminal stations features receiving from a plurality of terminal stations, on a single channel, a plurality of RF response signals, determining for the plurality of the received response signals a round trip delay, and thereby determining, for each of the response signals, a distance between the reader and the terminal station from which the response signal was received.
To one of ordinary skill in the art, there is thus a need for, and it would be highly useful to have a method and a corresponding system, for determination of RF tags, based on measuring the round trip delay. Moreover, to one of ordinary skill in the art, there is also a need for a method and a corresponding system, for determination of RF tags, based on measuring the round trip delay by using a single channel. Operation by a single channel simplifies the RFID system design and reduces the cost of implementation.
It is also desirable to have a collision overcoming method consuming minimal time that is almost not increasing with the number of tags participating in the collision.
Furthermore, it is desirable to have a collision overcoming method that is relatively simple, inexpensive to implement and does not require a plurality of readers in different locations.